The presence of the rash is pathognomonic, giving the diagnosis. Tests are done to:
A simple assessment of blood levels of glucagon can be made. They are elevated 10-20 times above normal in the presence of a glucagonoma.
A few other conditions can cause increased glucagon concentrations however they are readily distinguishable from glucagonoma from the history. The differential diagnosis includes:
Glucagonomas tend to be large (> 4 cm diameter, some are documented as being up to 35 cm). Therefore the imaging techniques of CT, MRI and ultrasound accurately detect such tumours. Very occasionally the presence of very small tumours will require the use of other investigation, such as somatostatin receptor scintigraphy. A radioactive-labelled analogue of somatostatin, indium-111 pentetreotide, is injected. This binds specifically to the somatostatin receptors of the tumour cells. X-rays then show up the area where the radiolabelled molecule is, therefore indicating where the tumour is.